1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method.
2. Description of the Related Art
A color drifting often occurs in a full color image formed by superimposing images in magenta, cyan, yellow, and black by a color image forming apparatus, degrading image quality of the full color image. Specifically, the color drifting occurs more often if a write optical system and an image carrier are arranged with respect to each color and images in different colors are thereby formed by different optical systems and image carriers like in a four-drum tandem type image forming apparatus. To correct the color drifting, a position-misalignment detection pattern is generally formed on a transfer belt with a sensor to read the position-misalignment detection pattern to detect the amount of a position (color) misalignment of different color images, and the position misalignment is corrected by adjusting a write timing or by using an optical-system correcting unit.
An image forming apparatus for correcting the color misalignment is disclosed in, for example, Japanese Patent Application Laid-Open No. 2003-228216. Specifically, the image forming apparatus includes a plurality of image forming units having image carriers to form toner images, a conveying unit that conveys a recording medium along with the image forming units, a transferring unit that sequentially transfers the toner images onto the recording medium, and a detecting unit that detects position information on the image transferred by the transferring unit. With this configuration, position-misalignment detection patterns for detecting a position misalignment between images formed by the image forming units are superimposed on one another onto one of the recording medium and the conveying unit, and the detecting unit detects a boundary of the position-misalignment detection patterns, so that at least one of the image forming units is controlled to correct the position misalignment based on the information detected by the detecting unit.
Furthermore, Japanese Patent No. 3266849 discloses another image forming apparatus having a function of correcting the color misalignment. Specifically, the image forming apparatus includes a plurality of photosensitive elements, an optical writing unit that writes different color data of an image on each of the photosensitive elements, and a developing unit that develops the data using corresponding color developer, and configured to form a full color image by sequentially transferring the developed images on the photosensitive elements to the transfer sheet conveyed by a transfer belt. At this state, the image forming apparatus further includes a pattern-image forming unit and a position-misalignment detecting unit. The pattern-image forming unit forms, on one of the transfer belt and a transfer sheet conveyed by the transfer belt, a measurement pattern image for each color that contains lines arranged in close vicinity of each other in the main-scanning direction. The position-misalignment detecting unit includes a slit plate and a position detecting unit on a transfer path corresponding to a position of conveyance of the measurement pattern image. The slit plate is integrated with slits of the same number as that of the lines in the measurement pattern image and with a width substantially equal to a line width. The position detecting unit includes an irradiation light source and detects the amounts of a transmitted light and a reflected light at the measurement pattern image on a slit portion for each slit. With this configuration, a space between the lines in the measurement pattern image is set so that each phase is shifted with respect to a space between slits in the slit plate.
Moreover, Japanese Patent No. 3518825 discloses still another image forming apparatus having the function of correcting the color misalignment. Specifically, the image forming apparatus includes a movable member, a plurality of image carriers, a correction-mark forming unit, a correction-value calculating unit, and a correcting unit. The movable member is conveyed at a predetermined speed and to which a recording sheet is adhered. The image carriers arranged along a conveyance direction of the recording sheet form latent images corresponding to image data by optical scanning of corresponding photosensitive drums, develop the latent images by different developing units, and transfer developed images onto the recording sheet on the movable member. The correction-mark forming unit transfers, on the movable member by using the image carriers, a mixed-color mark in which a plurality of marks are superimposed as a mark for correcting the color misalignment between different color images. The correction-value calculating unit detects a lightness pattern of the mixed mark transferred onto the movable member, and calculates a correction value of the color misalignment between different color images based on the phase of the lightness pattern. The correcting unit corrects the position misalignment between different color images based on the correction value.
However, in the conventional technologies described above, the amount of the color misalignment is measured based on the output from a sensor that reads the color-misalignment detection pattern formed on an image forming surface of the image carries that moves at a predetermined speed in the sub-scanning direction. Therefore, if the total length of the color-misalignment detection pattern increases, a detection time also increases, resulting in increasing a total processing time for measuring the amount of the color misalignment.